When Stella and her younger brother, Tom, move to their new London home, they become mystified by the disappearances of Harry, their elderly neighbour's dog. Where does he go? And why does he keep reappearing wet-through? Their quest to solve the riddle over the summer holidays soon leads to a boat buried under a grassy mound - and a tunnel that takes them to a secret lake. Who is the boy rowing towards them? Why is he so terrified? And whose are...